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Cathodoluminescence spectroscopy (CL) is a unique technique to probe optical modes at the nanoscale. The electric field surrounding a 10-30 keV electron beam dynamically polarizes matter, creating optical excitations over the 0-10 eV spectral range, that are then detected in the far field. CL can measure the dispersion of plasmonic and dielectric nanostructures at deep-subwavelength spatial resolution. So far, CL has probed the angle-dependent spectrum and polarization of nanoscale emitters. However, detecting the phase of the emitted plasmon scattering wavefronts has remained elusive.
Here, we introduce Fourier-transform CL holography as a method to determine the far-field phase distribution of scattered plasmonic fields. To do so, we measure the interference between two fields: (1) the electron-induced CL emitted by a plasmonic nanoscatterer and (2) a broadband reference field created by transition radiation induced by the same electron. From the angular interference patterns we directly reconstruct angle-resolved phase and intensity distributions. Taking the 6 (x-y-z) plasmonic electric and magnetic dipoles as a complete orthogonal set of scatterers we directly derive from the amplitude and phase data the relative strength and phase of all scattering dipoles, as they are excited by electron-beam.
We investigate the resonant scattering of 30 keV electron-beam excited surface plasmon polaritons (SPPs) off single-crystalline Ag nanocubes and find dominant scattering from the z-oriented electric dipole plasmon. In contrast, SPP scattering from nanoscale holes made in a Ag film induces an in-plane x-oriented electric dipole with the concomitant y-oriented magnetic dipole; both interfering in the far field creating a strongly beamed plasmon scattering distribution. Using a newly developed CL energy-momentum spectroscopy configuration we derive the phase of scattered fields as a function of frequency. The data are fully consistent with the plasmon polariton dispersion and the pi phase flip across the scattering resonance is directly observed in the measured phase fronts.
Fourier-transform CL holography opens up a new world of coherent light scattering and surface wave studies at nanoscale spatial resolution. It also opens up novel ways to investigate the temporal and spatial coherence of electron beam wavefonts and addresses fundamental questions regarding the collapse of the electron wavefunction as it excites surface plasmons.
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